460 CHEMICAL ENGINEERING area of the image overlaps this hole and extends half way to the edge of the focusing mirrors. The Fery pyrometer requires a smaller source than the fixed-focus instruments. The following table illustrates the size of source required for various sighting distances in order that the image cover the limiting diaphragm and extend about half way to the edge of the focusing mirrors, i.e., diameter of image = 4 mm. Errors to Which Radiation TABLE 17 Pyrometers are Subject.—Dust SIGHTING DISTANCE, DIAMETER OF SOUECE a,nd dirt allowed to accumulate CENTIMETERS CENTIMETERS upon ^ concave reflecting 70 o. 2i . , .. gQ £ 7 mirror may so decrease its re- JQQ 4 § flection coefficient as to develop 150 33 errors amounting to 100° or even 200 11.2 200°C. Frequently the dust can 300 16.8 be removed from the mirror by 500 28.5 carefully brushing with a camel's hair brush. The mirror may be removed from the instrument and washed, but this must be done with great care to insure that the delicate thermocouple or its mounting is not disturbed. The safest practice is to take all possible precautions to prevent dust from entering the instrument. Keep the case closed, or the front diaphragm of the fixed focus instruments plugged with a cork when not in use. As shown above, the simple theory of the radiation pyrometer states that the readings are independent of the sighting distances or size of source provided the source is larger than the minimum size of source demanded by the geometry of the instrument. Actually this ideal condition is not always realized. Some stray radiation is reflected down the walls of the telescope case. The walls become heated by the furnace and re-radiate to the couple. Limiting diaphragms are similarly heated and re-radiate to the couple. For these reasons a radiation pyrometer tends to read low the greater the sighting distance or the smaller the size of source. It is thus in general desirable to use a radiation pyrometer as nearly as possible in the same manner from day to day and to have it specially calibrated for such use. For example, if a 6-in. source is employed, decide upon a convenient sighting distance, say 4 ft., and always use this distance, and not sometimes 1 ft. and sometimes 5 ft. In the Fery pyrometer both the proper size of source and the proper focusing distances are secured by following the rule suggested above, viz. focus at such a distance that the image extends half way between the black hole and the outside edge of the focusing mirrors. Advantages and Disadvantages of Radiation Pyrometers.—For temperatures above 1,400 or 1,500°C. either a radiation pyrometer or an optical pyrometer must be employed. The optical pyrometer is capable of higher accuracy and is less, susceptible to errors than the radiation pyrometer. Smoke and dust affect the readings of both instruments but the radiation pyrometer is affected by the presence of cooler strata of carbon dioxide and other gaseous combustion products in the furnace. Carbon dioxide and water vapor absorb the heat rays and hence the radiation pyrometer will read too low when sighted through such gases. The main advantage of the radiation pyrometer is the fact that it can be made automatically recording. The recording mechanism is the same as that employed for ordinary thermocouples, discussed in the section on recording pyrometry. The radiation pyrometer is desirable for many processes of lower temperature where thermocouples cannot be conveniently installed. It is also useful in measuring the surface temperature of large ingots.